Insulin resistance in the peripheral tissues such as muscle and fat is associated with increased secretion of insulin by pancreatic β-cells. The secreted insulin promotes glucose utilization and inhibits production of glucose by the liver. However, the pancreatic β-cells often cannot sustain the increased production of insulin resulting in the eventual decrease of insulin production and glucose intolerance.
Insulin resistance is characterized, for example, by increased glucose concentration in the blood, increased insulin concentration in the blood, decreased ability to metabolize glucose in response to insulin, or a combination of any of the above. Insulin resistance is thought to predict possible later development of diabetic disease, such as Type 2 Diabetes. However, even in the absence of diabetes, insulin resistance is a major risk factor for cardiovascular disease (Despres, et al., N. Engl. J. Med 334:952-957 (1996)). The loss of insulin production in insulin resistance and diabetes results in increased blood glucose or hyperglycemia. Hyperglycemia in turn can contribute to long term illness such as nephropathy, neuropathy, and retinopathy.
Insulin resistance is also associated with abnormalities in glucose and lipid metabolism, obesity, kidney disease, high blood pressure and increased risk for cardiovascular disease. The association of insulin resistance with these other abnormalities is referred to as “Insulin Resistance Syndrome” or “Metabolic Syndrome” or “Syndrome X”. In particular, Metabolic Syndrome has been characterized as the co-occurrence of obesity (especially central obesity), dyslipidemia (especially high levels of triglycerides and low levels of high density lipoprotein cholesterol), hyperglycemia and hypertension. People with Metabolic Syndrome are at increased risk for diabetes or cardiovascular disease relative to people without the syndrome (Meigs, J. B., BMJ: 327, 61-62, (2003)).
Decreased expression of the insulin responsive glucose transporter, GLUT4, is seen in adipocytes in nearly all insulin resistant states in humans and rodents (Shepherd, P. R. and Cohn, B. B., N Engl. J. Med. 341:248-257 (1999)). However, the mechanism by which decreased expression of GLUT4 contributes to systemic insulin resistance has not been clear because adipose tissue contributes very little to total body glucose disposal.
Due to the association of insulin resistance with later development of diabetes and cardiovascular disease, and the prevalence of insulin resistance worldwide, the need exists for additional metabolic or endocrine targets for the development of treatments that alleviate or mitigate diseases associated with insulin resistance. A need also exists for additional detection/diagnostic methods of insulin resistance, Metabolic Syndrome and Type II diabetes to allow for the earliest possible intervention through life-style changes and/or medication.